Senate Democrats successfully blocked debate on Trade Promotion Authority in their chamber until they were promised a vote on Trade Adjustment Assistance.

A similar gambit faces longer odds in the House, but Congressional Progressive Caucus Co-Chairmen Raúl M. Grijalva of Arizona and Keith Ellison of Minnesota began Monday an effort to rally support among Democrats to force changes to the pending TAA legislation.

The TAA is designed to help U.S. workers displaced by trade agreements, and many progressive argue it's necessary if they're also going to give President Barack Obama latitude to negotiate a major trade deal with Pacific nations.

But Grijalva and Ellison, who are collecting signatures for a letter addressed to Speaker John A. Boehner and Minority Leader Nancy Pelosi, aren't impressed with the current TAA framework, which derives much of its funding from sequester cuts to Medicare.

"American workers rely on Trade Adjustment Assistance when trade deals send jobs overseas. While it is clear we need Trade Adjustment Assistance, it is not clear why the offset should be extended budget limitations on Medicare, as proposed," Grijalva and Ellison write in their letter, an early copy of which was obtained by CQ Roll Call. "More than 50 million seniors rely on Medicare; we should be investing in the trust fund, not using savings to fund other programs."

The two lawmakers are asking leadership not only to find another offset for TAA, but also to increase funding overall "to account for projected job losses due to big trade deals, and extent assistance to public sector workers who have lost their jobs."

With the overall anti-trade sentiment among House Democrats already so high, reservations about TAA don't make things easier for the members of Congress who want badly to get a deal to the president's desk.

Read the full letter here:

Dear Speaker Boehner and Leader Pelosi:
We write to urge you to fund a more robust Trade Adjustment Assistance program that is not offset by cuts to critical social programs like Medicare.
American workers rely on Trade Adjustment Assistance when trade deals send jobs overseas. While it is clear we need Trade Adjustment Assistance, it is not clear why the offset should be extended budget limitations on Medicare, as proposed. More than 50 million seniors rely on Medicare; we should be investing in the trust fund, not using savings to fund other programs.
We ask that the current Trade Adjustment Assistance legislation change in two ways. We should find an offset to Trade Adjustment Assistance that does not cut from critical programs that working families rely on. We should also increase funding for Trade Adjustment Assistance to account for projected job losses due to big trade deals, and extend assistance to public sector workers who have lost their jobs.
Trade Adjustment Assistance supports displaced workers and Medicare provides America's seniors the healthcare they need. We look forward to working with you to fund a Trade Adjustment Assistance program that leaves no one behind.